The invention relates to a lifting hook arranged to grab an object to be lifted by a cylindrical lifting surface formed thereto, the lifting hook having a hook shank, a hook part joining the hook shank and a locking device arranged to lock the lifting hook to the object to be lifted, a mechanical control device being mounted above the hook part to co-operate with the upper surface of the lifting surface for guiding the hook part to a lifting position onto the lower surface of the lifting surface. The invention particularly relates to a hook meant for handling jumbo paper rolls in a paper mill, in which case two hooks are usually provided. The hooks are attached to the ends of a load-carrying beam designed for handling paper rolls and configured to engage with cylindrical lifting surfaces of cylindrical reeling shafts of the paper rolls. The hook is also suitable for handling other objects provided with a cylindrical lifting surface.
Prior art lifting hooks have been disclosed for example in publications U.S. Pat. No. 2,577,790 A, U.S. Pat. No. 5,114,200 A and JP 2001354388.
The lifting hooks of a load-carrying beam of a crane used in a paper mill for lifting and moving a paper roll are first guided to cylindrical lifting surface grooves formed to the ends of the reeling shafts of the paper roll, then the hooks are guided into the grooves, making sure that they engage with the lifting surface grooves of the shafts from below, the hooks being then locked to the lifting surface grooves and hence the paper roll may be lifted and transferred. This may take place either automatically or by manual control.
A problem with the paper roll handling described above is that particularly in connection with an automated lifting control, the lifting hook does not engage properly with the lifting surface groove, the hook drifts outside the groove or an erroneous hook locking confirmation is received even if locking of the hook to the proper locking point had not taken place. If the paper roll or one of its ends then comes off from one or both of the hooks in connection with the lifting, serious damage may be caused. Typically, if a roll falls onto its support stand from a height more than 10 cm, for example, the expensive reeling shaft of the paper roll will become twisted and unusable. On the other hand, if the roll falls onto the floor of the paper mill, the entire floor structure may suffer significant damages on a large area.
Automated lifting control has typically been carried out using information provided by location sensors mounted to the lifting hook and possibly to the reeling shaft of the paper roll as well, which has not been completely reliable in all situations. It has therefore usually been necessary to ensure the fastening manually or visually, which in practice often means that the crane operator walks to the spot and confirms the situation.
In connection with manual lifting control the crane operator usually fastens and locks the lifting hooks to the paper roll by hand, which always requires walking back and forth between the crane and the paper rolls.
It is clear from the above that without reliable lifting hook control, fastening and locking paper rolls may be expected to fall off and to cause considerable costs due to damages. If this risk is to be avoided, as it nowadays always is, the handling of paper rolls will suffer from unnecessary delays because of the obligatory manual and visual control checks made on foot.